1. Field of the Invention
This invention relates to liquid crystal displays and, more particularly, to the improvement of image quality in reflection type liquid crystal displays using no back light.
2. Description of the Related Art
Liquid crystal displays are devices in which a liquid crystal layer having a thickness of the order of 5 .mu.m is sandwiched between two glass plates having electrodes thereon and images are displayed by controlling the movement of the liquid crystal molecules. Accordingly, they can provide much thinner display units than CRTs.
In a common TN liquid crystal display, images are displayed by disposing a back light source on the outside of the liquid crystal display and controlling the liquid crystal molecules so as to transmit or absorb light from the back light source. This is called a transmission type liquid crystal display. The transmission type liquid crystal display has the disadvantage that the use of a back light source causes an increase in power consumption.
One means for solving this problem is provided by a reflection type liquid crystal display in which external light is introduced and reflected instead of using back light. When a reflector is disposed on the outside of a TN liquid crystal display, this liquid crystal display has the disadvantages that the displayed image is generally dark and the displayed characters or the like produce shadows. The reason for the darkening is that the TN liquid crystal display requires polarizers and, therefore, the amount of external light introduced is reduced to one half. The reason why the displayed characters or the like produce shadows is that, since a reflector is disposed on the outside, there is a great distance between the display screen and the reflective surface and, therefore, the display screen is mirrored in the reflector.
A liquid crystal display of the phase transition guest-host (GH) type has low contrast and is seldom used as a transmission type liquid crystal display using back light. However, when this is used as a reflection type liquid crystal display, it is more disadvantageous than the TN liquid crystal display. That is, since no polarizer is required, the amount of light introduced is greater and the displayed image becomes brighter. Moreover, since a reflector may be formed within the liquid crystal layer, the displayed characters or the like produce no shadow.
The operating principle of a liquid crystal display of the GH type is described with reference to the schematic section of FIG. 7. As shown in FIG. 7, this liquid crystal display includes, on the panel surface side, a glass substrate 1 on which an ITO layer 2 and a polyimide layer 5 serving as a protective layer are formed in that order. On the opposed board side, it also includes a glass substrate 1 having a reflector layer 6 and a polyimide layer 5 formed thereon. A GH liquid crystal layer 7 is sandwiched therebetween to construct the liquid crystal display. In this GH liquid crystal layer 7, several percent of dichroic dye molecules 3 are mixed with liquid crystal molecules 4. These dichroic dye molecules 3 do not change their direction under the influence of an electric field. However, since dichroic dye molecules 3 have a size almost equal to that of liquid crystal molecules 4, they behave in the same manner as the large number of liquid crystal molecules 4. Moreover, dichroic dye molecules 3 transmit or absorb light according to their direction. That is, in the absence of an applied voltage as shown in FIG. 7(a), dichroic dye molecules 3 are horizontally oriented, and exhibit a spiral configuration in the presence of a chiral dopant. If the angle of twist is greater than 90 degrees, any type of polarized light 19 entering the panel from its surroundings is absorbed, so that the panel looks black. On the other hand, in the presence of an applied voltage as shown in FIG. 7(b), dichroic dye molecules 3 are vertically oriented in conformity with the orientation of the liquid crystal molecules. Consequently, incident light is transmitted by GH liquid crystal layer 7, is reflected by the upper reflector layer 6, and exits as outgoing light 20, so that the panel looks white.
As reflector layer 6, there is commonly used a metal film formed by depositing a metal having high reflectivity (e.g., aluminum or silver) on substrate 1 by sputtering or vapor deposition.
The circuit of an active matrix-driven liquid crystal display actually used to display images is described with reference to FIG. 8. In FIG. 8(a), image signals are applied to signal lines X1, X2, . . . , Xn. At each of the intersections of these signal lines and scanning lines Y1, Y2, . . . , Yn, a thin-film transistor (TFT) 12 is connected. This TFT 12 is connected to a pixel electrode, so that the pixel electrode and the counter electrode, together with a liquid crystal sandwiched therebetween, constitute a pixel electrode capacitance 13. As shown in FIG. 8(b), drive pulses Z1, Z2, . . . , Zn are successively applied to scanning lines Y1, Y2, . . . , Yn. Taking scanning line Y1 as an example, TFTs 12 connected to scanning line Y1 are in the conducting state during a period in which drive pulse Z1 has a voltage of 20 V. As a result, the electric potential of the image signals applied to the signal lines is written into pixel electrode capacitance 13 constituted by the pixel electrodes and counter electrode 14 with the liquid crystal sandwiched therebetween. The period in which drive pulse Z1 has a voltage of 20 V is equal to 1/60 n second which is obtained by dividing the screen rewriting time, i.e., 1/60 second by the number (n) of the scanning lines. Next, TFTs 12 connected to scanning line Y1 are in the non-conducting state during a period in which drive pulse Z1 has a voltage of 0 V. Thus, the electric potentials of the image signals written into pixel capacitance 13 are retained until the voltage of drive pulse Z1 is increased to 20 V in the next scan. In this manner, an image is displayed. TFTs of the type commonly used in active matrix-driven liquid crystal displays are forward-staggered TFTs. These forward-staggered TFTs can be manufactured by two patterning steps and are frequently used because of their ease of manufacture.
A process for fabricating an active matrix-driven liquid crystal display by using these TFTs is described below. First of all, the method of making a TFT board is explained with reference to FIG. 9.
A glass substrate 1 is provided as shown in FIG. 9(a) and an ITO layer 2 is deposited thereon by means of a sputtering apparatus as shown in FIG. 9(b). A resist is applied thereto, exposed to light with a suitable exposure apparatus through a mask having patterns for defining signal lines and pixel electrodes, and then developed to leave the resist having, for example, the patterns of signal line X and pixel electrode 8. Next, ITO layer 2 is etched by using the patterned resist as a mask to form signal line X and pixel electrode 8. Thereafter, the resist is stripped off as shown in FIG. 9(c). Subsequently, after an ohmic contact layer is formed only on the patterned regions of ITO layer 2 by plasma treatment in a plasma CVD apparatus, an amorphous silicon layer 9 and a silicon nitride layer 10 are deposited thereon by means of a plasma CVD apparatus and a chromium layer 11 is deposited thereon by means of a sputtering apparatus as shown in FIG. 9(d). A resist is applied thereto, exposed to light with an exposure apparatus through a mask having patterns for defining scanning lines, and then developed to leave the resist having the patterns of the scanning lines. Next, chromium layer 11, silicon nitride layer 10 and amorphous silicon layer 9 are etched by using the patterned resist as a mask, and the resist is then stripped off [FIG. 9(e)]. Finally, a polyimide is applied with a printer and then rubbed to obtain a TFT board as shown in FIG. 9(f).
Next, the method of making a counter electrode board is illustrated in FIG. 10. A glass substrate 1 is provided [FIG. 10(a)] and a reflector layer 6 consisting of aluminum or silver having high reflectivity is deposited thereon by means of a sputtering apparatus [FIG. 10(b)]. Subsequently, a polyimide is applied with a printer and then rubbed to obtain a counter electrode board as shown in FIG. 10(c).
Then, the TFT board and the counter electrode board which have been made in the above-described manner are bonded with a predetermined gap therebetween. First of all, a sealant is printed on the film-bearing surface of the TFT board by means of a seal printer. This sealant functions as an adhesive for bonding the two boards. In order to secure a predetermined gap between the two boards, a gapping material in particulate form is incorporated in the sealant. Then, the TFT board and the counter electrode board are bonded with the sealant so that the film-bearing surfaces thereof face each other.
Finally, using a liquid crystal injector, a GH liquid crystal is injected into the gap between the two boards so bonded to fabricate an active matrix-driven reflection type liquid crystal display. A plan of this liquid crystal display is shown in FIG. 11. As shown in this figure, signal lines X1, X2, . . . , Xn and scanning lines Y1, Y2, . . . , Yn are disposed crosswise on the TFT board. TFTs 12 are formed in the vicinity of the respective intersections of the signal lines and the scanning lines. A schematic section taken along line A-B in FIG. 11 is shown in FIG. 12, and a schematic section taken along line C-D in FIG. 11 is shown in FIG. 13.
In this active matrix-driven reflection type liquid crystal display, an arbitrarily chosen voltage is always applied to the signal lines and the scanning lines, so that the portions of the liquid crystal above the signal lines and the scanning lines are in the light-transmitting state. However, as shown in FIG. 12, chromium layer 11 forming the scanning lines does not transmit light and, therefore, light 19 striking on this region does not pass through chromium layer 11. In contrast, as shown in FIG. 13, ITO layer 2 forming the signal lines and the pixel electrodes is transparent and hence transmits light. Consequently, light is transmitted not only in the region of pixel electrode B to which a voltage is applied, but also in the regions of signal lines X1 and X2. This light is reflected by the counter electrode board and exits to the outside (as indicated by arrows 21).
In conventional panel structures, an arbitrarily chosen voltage is always applied to transparent electrodes forming signal lines, as described above. Consequently, the portions of the liquid crystals are oriented under the influence of the resulting electric field and are in a partially transmitting state. The light striking on these regions is reflected by a reflector layer and exits to the outside. That is, a reduction in contrast due to the leakage of light through the signal lines has posed a problem. Moreover, when a white spot is displayed on the screen by switching on, for example, the TFT formed in the vicinity of the intersection of signal line X2 and scanning line Y2, as shown in FIG. 14, a white stripe is displayed in the region of the signal line which is also in the transmitting state, and makes the image difficult to see. The occurrence of such a phenomenon (what is so-called "cross talk") has also posed a problem.